The cross-linking, or curing, of polymers has been known for many years. Cross-linking, or curing, creates a three-dimensional structure, covalently bridging the polymer chains, that in general provides strength and stability to the polymer. The cross-linking of polymer systems is typically carried out by means of radiation (e.g. electron beam radiation) or by addition of suitable curing agents, e.g. the addition of sulphur in the vulcanization of rubber.
Curable compositions comprising a 1,5-enediyne curing agent have been described in WO 2011/076652 (SOLVAY SOLEXIS SPA) 30 Jun. 2011 which discloses a curable composition comprising:
(a) at least one polymer (P); and
(b) at least one curing agent of formula (I):
wherein each R in formula (I), equal to or different from each other, is independently selected from the group consisting of hydrogen; halogen (e.g. F, Cl, Br, I); C1-C20 alkyl, linear or branched, optionally substituted and/or optionally fluorinated; C1-C20 oxyalkyl, linear or branched, optionally substituted and/or optionally fluorinated; (per)fluoropolyether chain; aromatic or heteroaromatic radical, monocyclic or polycyclic, optionally substituted and/or optionally fluorinated; —SiR13, —(R12SiO)bR1, —PR12 wherein each R1, equal to or different from each other, is independently selected from the group consisting of hydrogen, C1-C20 alkyl, linear or branched, optionally substituted and/or optionally fluorinated and wherein b is an integer of at least 1; and wherein A1 and A2, equal to or different from each other, are each independently selected from the group consisting of hydrogen; halogen; C1-C20 alkyl, linear or branched, optionally substituted and/or optionally fluorinated; C1-C20 oxyalkyl, linear or branched, optionally substituted and/or optionally fluorinated; (per)fluoropolyether chain; —(R12SiO)bR1 wherein R1 and b are as defined; aromatic or heteroaromatic radical, monocyclic or polycyclic, optionally substituted and/or optionally fluorinated; and wherein A1 and A2 may be comprised in an alkyl or aromatic cyclic structure, optionally substituted and/or optionally fluorinated. Polymer (P) may be selected from hydrogenated and fluorinated polymers. The ethynyl groups on adjacent carbon atoms in formula (I) are known to dimerize upon application of heat to form an aromatic ring having a 1,4-diradical. While not being bound by theory, it is believed that the 1,4-diradical may promote the cross-linking or curing process via a Bergman cyclization reaction such as the one disclosed by Warner et al. in Science, 268, (1995), pp. 814-816.
The curable compositions disclosed WO 2011/076652 (SOLVAY SOLEXIS SPA) 30 Jun. 2011 may additionally contain additives capable to improve the dispersion of the curing agent of formula (I) into polymer (P). When polymer (P) is a fluorinated polymer, suitable dispersing additives are described as compounds comprising a (per)fluoropolyether chain said chain comprising either aromatic pendant groups and/or aromatic terminal groups, said aromatic groups being optionally fluorinated.
The Applicant has now surprisingly found that when certain fluorinated aromatic compounds are added to a curable composition comprising a fluorinated polymer and a curing agent of formula (I) as defined above the temperature at which thermally induced curing of the composition takes place can be reduced. Additionally, the elastic modulus of the resulting cured article is generally higher than the elastic modulus of cured articles obtained from the curable compositions disclosed in WO2011/076652.